Projected capacitive touch panel systems are well known. Such systems are typically employed in touchscreens of electronic devices, such as smart phones, handheld gaming devices, global positioning system (GPS) devices, point-of-sale credit/debit card machines, and so forth. Typically, projected capacitive touch panel systems utilize a grid of sensing points to sense changes in self capacitance (e.g., between a sensor and ground) or mutual capacitance (e.g., between two sensors) as a result of a touch input caused by a user touching an area of the panel. The grid sensors typically coincide with pixel locations where the touch panel system forms part of a touchscreen display.
An exemplary touch panel system 100 is illustrated in FIG. 1. The touch panel system 100 includes a grid or matrix of sensing points 102 encased within a housing and covered by a protective dielectric lens. In the illustrated system 100, forty-eight (48) sensing points 102 are shown. The sensing points 102 are capacitive in nature and may be formed by overlapping conductors in a multilayer implementation or arranging conductors adjacent to one another in a single layer implementation. An excitation signal is applied to one or more excitation nodes 104 (X0-X7) and an output signal is detected at one or more output nodes 106 (Y0-Y5). The output signals are proportional to the sensing point capacitances.
For example, the charge of a capacitor with capacitance C at time t can be determined by the equation:Q=CVe−t/τ, where                τ=RC;        R is the resistance associated with the capacitor (e.g., inherent and terminal resistances); and        V is the voltage of the capacitor when fully charged.Therefore, changes in the capacitance of a sensing point 102 can be detected by measuring the change in the discharge time of the sensing point 102. The discharge time is measured by measuring the output voltage decay as a function of time after application of an input signal burst. In a typical capacitive touch panel system 100, excitation signals are sequentially applied to each excitation node 104 and timers are used to measure the discharge times at the output nodes 106. For example, an excitation signal burst is applied to excitation node X0 and timers are used to measure the discharge times at all the output nodes 106 (Y0-Y5) to determine the capacitances at the sensing points 102 defined by excitation node X0 and the output nodes 106 (Y0-Y5). Excitation signals are then sequentially applied to the other excitation nodes 104 (X1-X7) and the discharge times at the output nodes 106 (Y0-Y5) are measured to determine the capacitances at the remaining sensing points 102. In other words, the sensing points 102 are effectively scanned repeatedly to detect variations in capacitance. Variation in the determined capacitances at one or more sensing points 102 may indicate a touch input in the area of the sensing points 102.        
The application of a touch input signal to the touch panel system 100 may have different effects depending on the arrangement of the sensor grid. FIGS. 2 and 3 illustrate the effect of a touch input on electric fields and, therefore capacitance, depending on whether the sensing points 102 are configured to detect self-capacitance or mutual capacitance. For example, as illustrated in FIG. 2, the sensing points 102 may be arranged as a grid of sensors 203 that detect self-capacitance relative to ground 204. As noted above, the sensors 203 are typically protected by a flexible dielectric lens 201 that is touched by a user's finger 209 or an instrument, such as a stylus, pencil, toothpick, or other device. In operation, an excitation or drive signal 205 is supplied as a burst to the sensor 203 by an integrated circuit (IC) or other source. The drive signal is then removed and the amount of time required for the voltage of the sensor 203 to decrease to zero or some predetermined threshold is measured as the discharge time. As detailed above, the discharge time is directly related to the capacitance of the sensor 203.
In the case of a sensor 203 configured to detect self-capacitance, an electric field is created between the sensor 203 and ground 204 upon excitation of the sensor 203 by the excitation signal 205. The electric field includes a quantity of electric field lines 207 or flux between the charged node or plate of the capacitive sensor 203 and ground 204. When a touch input is applied by a user's finger 209, more electric field lines 211 are added from the sensor 203 to ground 204 due to the capacitive nature of the human body. Such additional electric field lines 211 result in an increase in the effective capacitance as detected by the sensor 203. The increase in capacitance causes a resulting increase in the discharge time of the sensor because, as noted above, the discharge rate of a capacitor varies as an inverse function of the capacitance (e.g., as capacitance increases, τ increases, the exponential discharge rate decreases, and the duration of time required to discharge the capacitor increases).
By contrast, a touch input has the effect of decreasing the effective capacitance where the sensing points 102 are configured to detect mutual capacitance. For example, as illustrated in FIG. 3, each sensing point 102 may be configured as two parallel sensors 301, 303 that sense the mutual capacitance between them. In this case, an IC excitation signal 305 applied to sensor 301 is received in attenuated form as a received signal 307 at sensor 303 based on the capacitance between the two sensors 301, 303. The difference in potential between the two sensors 301, 303 in the presence of the applied excitation signal 305 causes an electric field to be generated between the two sensors 301, 303. As illustrated in FIG. 3, the electric field includes electric field lines 309 or flux emanating from the charged sensor 301 and terminating at the receiving sensor 303. If a user touches the dielectric lens 201 near the sensors 301, 303, some of the electric field lines 311 are redirected to the user's finger 209 due to the inherent capacitance between the user and ground. The redirection of the electric field lines 311 causes an effective reduction in the capacitance between the two sensors 301, 303. As is known, the impedance between the two sensors 301, 303 is inversely proportional to the capacitance between the two sensors 301, 303. Therefore, as the capacitance decreases due to the presence of the touch input, the impedance of the capacitor increases and the level of the signal 307 received at the receiving sensor 303 decreases.
In an ideal environment, sensing points 102 of touch panel systems 100 would only detect signals and capacitance changes resulting from touch inputs. However, due to the presence of the housing and various electronic components of electronic devices that include touch panel systems 100, such systems 100 are required to distinguish touch inputs from various noise sources. For example, in a touchscreen display, a display panel is positioned in close proximity to the touch panel so that information can be displayed to the user as part of the touch panel user interface experience. An exemplary touchscreen display configuration is illustrated in FIG. 4. In the illustrated touchscreen display, a display panel 401 is positioned in close proximity below the touch panel system. In this case, the dielectric lens 201 would most likely be transparent so that the information displayed on the display panel 401 could be seen through the lens 201. The touchscreen display gives the user the impression that he or she is selecting areas on the display panel 401, when instead the user is actually selecting areas of the touch panel system that correspond to the displayed areas of the display panel 401. Software within a processing unit of the electronic device employing the touchscreen display maps the areas of the touch panel system to the information displayed on the display panel 401 to achieve the function or result desired by the user based on the user's touch input. Thus, accurately recognizing where the user touched the touch panel system is critical to proper operation of the touch panel system or a touchscreen display incorporating it.
Electronic components, such as display panels, generate spectral noise during operation. Such noise may be static (time-invariant) and/or may vary over time depending on the output of the electronic component. For example, the noise produced by a display panel, such as a liquid crystal display (LCD) panel, varies over time depending upon the colors and images displayed by the display panel, the refresh rate of the panel's pixels, the resolution of the display panel, and the display panel's backlight intensity. Placement of a noise-producing, electronic component in close proximity to capacitive sensors 203, 301, 303, such as those employed in a capacitive touch panel system, creates parasitic capacitance between the noise-producing component (e.g., a display panel) and the touch panel sensors 203, 301, 303, which in turn creates time-varying noise in the touch panel system. FIG. 5 provides a graphical illustration of the spectral noise detected by a capacitive touch panel system due to noise produced by a display panel positioned in close proximity and directly below the touch panel system. As can be seen from the graph 500, the amplitude of the noise 503 is only about three decibels (3 dB) below the amplitude of the touch input 501. Thus, the signal-to-noise ratio illustrated in FIG. 5 is only 3 dB. The signal-to-noise ratio would likely be even worse in touchscreen displays in which the touch panel system is integrated directly into the display panel. Much higher signal-to-noise ratios are desired to improve touch input resolution and facilitate higher sensing point scanning speeds.
As noted above, noise produced by a display panel typically comes in two forms—static (time-invariant) noise and time-varying noise. Prior art solutions have addressed filtering and other compensation for static noise components to assist in improving signal-to-noise ratios. However, prior art solutions have not adequately accounted for time-varying noise, which may be a significant noise source especially in touchscreen displays.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated alone or relative to other elements to help improve the understanding of the various embodiments of the present invention.